diff --git a/services/baremetal-builder/default.nix b/services/baremetal-builder/default.nix
index 581a0e8..ee3dbdf 100644
--- a/services/baremetal-builder/default.nix
+++ b/services/baremetal-builder/default.nix
@@ -146,14 +146,23 @@ in
     };
 
     systemd.services.hydra-gc = {
+      wantedBy = [ "multi-user.target" ];
       description = "Nix Garbage Collector";
-      script = "exec ${config.nix.package.out}/bin/nix-store --gc --store /mnt";
+      script = ''
+        while : ; do
+          percent_filled=$(($(stat -f --format="100-(100*%a/%b)" /)))
+          if [ "$percent_filled" -gt "85" ]; then
+            ${config.nix.package.out}/bin/nix-store --gc --max-freed 100G --store /mnt
+          else
+            break
+          fi
+        done
+      '';
       serviceConfig.Type = "oneshot";
       serviceConfig.User = "builder";
     };
     systemd.timers.hydra-gc = {
-      timerConfig.OnStartupSec = "4h";
-      timerConfig.OnUnitActiveSec = "20h";
+      timerConfig.OnUnitInactiveSec = "10min";
       wantedBy = [ "timers.target" ];
     };
     systemd.timers.hydra-gc.timerConfig.Persistent = true;